Presently, many internal combustion engines are compression ignition engines fuelled by diesel fuel. In such engines, liquid fuel is injected directly into the combustion chamber so the fuel must be pumped to an injection pressure greater than the pressure inside the combustion chamber when the fuel is being introduced, to overcome the so-called “in-cylinder pressure”. In a diesel engine, the peak in-cylinder pressure is typically less than 20 MPa (about 3,000 psi). However, injection pressures significantly higher than the in-cylinder pressure are desirable to assist with greater atomization of the liquid fuel, since this can lead to more efficient combustion. For example, it is not uncommon for a modern diesel engine to employ injection pressures of at least about 140 MPa (about 20,000 psi) with some engines employing diesel injection pressures as high as 220 MPa (about 32,000 psi). At such high injection pressures, because the pressure differential between injection pressure and in-cylinder pressure is so large, fluctuations in the in-cylinder pressure have little impact on the amount of fuel being injected into the combustion chamber.
Because diesel fuelled internal combustion engines still generate a considerable amount of pollutants such as oxides of nitrogen (NOx) and particulate matter (PM), recent developments to reduce emissions have been directed to substituting some of the diesel fuel with gaseous fuels such as natural gas, pure methane, ethane, liquefied petroleum gas, lighter flammable hydrocarbon derivatives, hydrogen, and blends of such fuels. Gaseous fuels are generally defined herein as fuels that are gaseous at atmospheric pressure and zero degrees Celsius. Unlike liquid fuels, gaseous fuels can be injected into an engine's combustion chamber at a lower injection pressure because no extra energy is required for fuel atomization.
An advantage of substituting a gaseous fuel for diesel fuel is that the selected gaseous fuel can be one that burns cleaner than diesel fuels, and if a gaseous fuel is injected directly into the combustion chamber late in the compression stroke, with similar timing to when diesel fuel is injected in a conventional compression ignition engine, the high efficiency and high torque normally associated with conventional diesel engines can be preserved.
Another advantage of gaseous fuels is that, as a resource, such fuels are more widely distributed around the world and the amount of proven reserves of natural gas is much greater, compared to proven oil reserves.
Gaseous fuels can also be collected from renewable sources such as vent gases from garbage dumps and sewage treatment plants. Hydrogen can be produced with electricity generated from renewable sources such as wind power and hydro-electric dams.
As noted above, the injection pressure for gaseous fuels can be lower than the injection pressure normally used for liquid diesel fuels because no extra energy is needed for atomizing the fuel. A lower injection pressure for gaseous fuels is also desirable because employing higher injection pressures would increase the parasitic load on the engine system. To achieve efficient combustion, the injection pressure for a gaseous fuel need only be sufficient to overcome in-cylinder pressure with enough energy to disperse the gaseous fuel within the combustion chamber and to introduce the desired amount of fuel within a desired injection on-time. Different engines have different compression ratios and different in-cylinder pressure profiles but by way of example, if an engine has a maximum in-cylinder pressure during the compression stroke of about 20 MPa (around 3,000 psi), an injection pressure of about 30 MPa (4,350 psi) can be sufficient for injecting the desired amount of fuel and achieving an efficient combustion. However, when the injection pressure is this close to the in-cylinder pressure, the differential pressure between the injection pressure and the in-cylinder pressure is much lower, compared to that with diesel-fuelled engines and variations in the in-cylinder pressure and the injection pressure can influence the amount of gaseous fuel that is injected into the combustion chamber, which in turn influences other factors such as combustion efficiency, engine performance and operational consistency.
The in-cylinder pressure for gaseous fuelled engines can have an even stronger influence on the fuelling rate when the gaseous fuel injection valve design employs the fuel pressure to assist with operation of the valve. For example, in known injection valve designs with an inward opening needle, it is common to use the pressure of the fuel inside the fuel injection valve to act on a shoulder feature of the valve needle to provide a portion of the opening force. In a diesel fuel injection valve, since the pressure of the diesel fuel is much greater than the in-cylinder pressure, changes in the in-cylinder pressure have no noticeable effect on the speed at which the needle of such injection valves moves from the closed to open positions. However, with a similarly designed fuel injection valve for a gaseous fuel that is introduced at a much lower injection pressure, because of the smaller differential between the in-cylinder pressure and the fuel pressure, changes in the in-cylinder pressure can influence the speed at which the valve needle moves from the closed position to the open position. That is, for a gaseous fuel injection valve, higher in-cylinder pressures can increase the valve opening speed, which can result in a higher fuel mass flow rate for a given gaseous fuel injection time. Likewise, with outward opening valves, also known as poppet-style valves, again because of the lower differential between the fuel pressure and the in-cylinder pressure, variations in the in-cylinder pressure can influence the movement of the valve when fuel pressure is relied upon to provide part of the actuating force for the valve. For example, with these types of valves higher in-cylinder pressures can hinder the opening of the valve, resulting in less fuel being introduced into the combustion chamber.
Co-owned U.S. Patent Application Publication No. 2009/0084348 describes a method of calculating a corrected pulse width by applying at least one correction factor to a baseline pulse width determined from a fuelling command. The baseline pulse width is first corrected by multiplying it by the in-cylinder pressure correction factor determined from a lookup table that inputs the timing for start of injection and the intake manifold pressure and then it is further corrected by a rail pressure correction factor. While instrumentation exists to measure in-cylinder pressure directly, such instrumentation is expensive, more suited to research purposes and is not considered economical, practical and reliable enough for large-volume commercial use. Therefore, in-cylinder pressure during an injection event is typically approximated by reconstruction from indirect measurements of parameters that correlate to in-cylinder pressure such as the intake manifold pressure and temperature, exhaust manifold pressure, engine speed and the timing for start of injection.
While previously known solutions have improved the accuracy of metering a fuel into the combustion chamber of a direct injection internal combustion engine, because in-cylinder pressure is approximated by indirect measurements, controlling the injection time of the gaseous fuel can be further improved by increasing the number of parameters that are associated with the adjustments made to injection on-time. The problem with adding more parameters is that this can add to the complexity of the determination of such adjustments, which need to be made quickly in real-time to be practical and effective, given the speed at which engines operate. In other words, there is a need for an effective and efficient method to more accurately determine the injection on-time and the amount of fuel metered into the combustion chamber of a direct injection gaseous-fuelled internal combustion engine based on several parameters.